Extensive sucrose cycling through the fructan pool of a C$_3$ grass across a 200 to 800 μmol mol$^{−1}$ atmospheric CO$_2$ gradient

Fructan stores constitute the most important reserve carbohydrate pools of cool season (C$_3$) grasses, but it is largely unknown how the variation of atmospheric CO2 concentration ([CO$_2$]) between past and projected future levels affects their function in source (photosynthesis)–sink (growth and respiration) relationships. This study addressed this question with perennial ryegrass (Lolium perenne L.) grown at [CO$_2$] of 200, 400, or 800 μmol mol$^{−1}$ with growth-limiting nitrogen fertilization. Sixty-six days-old vegetative stands were labelled with $^{13}$CO$_2$/$^{12}$CO$_2$ mixtures for 7 days and the $^{13}$C-tracer dynamics in whole-shoot sucrose, fructans, glucose, and fructose pools evaluated with a 4-pool compartmental model of central carbohydrate metabolism. Increasing [CO$_2$] from 200 to 800 μmol mol$^{−1}$ increased shoot mass 1.6-fold and fructan concentration from 35% to 50% of dry weight, while decreasing the nitrogen nutrition status of stands. Conversely, [CO$_2$] had no effect on the half-lives of water-soluble carbohydrate pools (fructans, ∼7.7 days; sucrose, glucose, and fructose, 2.3 to 4.5 h). Carbon cycling through the fructan pool was enhanced by [CO$_2$], increasing the mean residence time of carbon in the carbohydrate system from 9 to 14 d between 200 and 800 μmol mol$^{−1}$ [CO$_2$]. ... mehrSucrose resynthesis from breakdown products of fructans (fructose) was extensive and corresponded to 65% and 113% of concurrent sucrose neo-synthesis (or sucrose consumption in growth and respiration) at 200 and 800 μmol mol$^{−1}$ [CO$_2$], respectively. These results imply a strong constitutive buffering of sucrose availability in the source–sink system of perennial ryegrass (and likely most other C$_3$ grasses) by fructan metabolism.